Adjustable dockboards are mounted within a pit or depression in the loading dock and include a ramp which is hinged at its rear edge to the frame or supporting structure of the dockboard and is movable between a downwardly inclined position and an upwardly inclined position. An extension lip is pivotally connected to the forward edge of the ramp and can be moved between a downwardly hanging pendant position and an extended position where it forms an extension to the ramp and can span the gap between the ramp and the bed of a carrier or truck.
In an upwardly biased dockboard, a spring assembly is utilized to bias the ramp to the upwardly inclined position. In a dockboard of this type, a holddown mechanism is employed to retain the ramp in any position within the working range. When the holddown mechanism is manually released, the spring assembly will urge the ramp to an upwardly inclined position.
U.S. Pat. No. 3,117,332 illustrates a common form of counterbalancing mechanism utilizing an extension spring. As illustrated in that patent, the rear end of the extension spring is connected to the frame or supporting structure of the dockboard, while the forward end of the spring is connected to an arm that is also pivoted to the frame. The forward end of the arm carries a roller which rides on a curved cam surface as the spring elevates the ramp to its inclined position. The cam is designed with a curvature such that the counterbalancing force will be slightly in excess of the force of the ramp at all elevations of the ramp.
The use of the cam in the counterbalancing mechanism as disclosed in U.S. Pat. No. 3,117,332, has certain disadvantages. The engagement of the roller with the cam surface provides substantial frictional resistance to elevation of the ramp and thereby requires a larger spring force to elevate the ramp. Increasing the counterbalancing spring force makes the ramp more difficult to walk down by the operator to effect engagement by the extended lip with the truck bed. Further, the roller is subject to considerable wear which requires periodic maintenance and replacement.
Compression springs have also been utilized in the past to provide the counterbalancing action for raising the ramp. As disclosed in U.S. Pat. No. 3,528,118, a compression spring is mounted on the frame in a fore-to-aft direction and the rear end of the compression spring is connected through a crank arm to the rear end of the ramp. With this type of counterbalancing mechanism, on release of the holddown, the force of the spring will act through the crank arm to pivot the ramp to the upwardly inclined position.
Because of the type of linkages involved, the counterbalancing springs, as used in the past, have generally operated in the range of 30% to 40% of their potential energy output and thus have not taken full advantage of the total available working range.